Hybrid and printed circuit technologies have existed for many years, however the trend in electronics is toward surface mount manufacturing. Surface mounted components are rapidly taking the place of conventional through hole mounted components for many applications because of the more compact arrangement and the ease of automated mounting that is possible with such components. Surface mounted components differ from through hole components in that the leads of a surface mount component are attached to the surface of the mounting substrate rather than protruding through the substrate as with through hole component leads. In recent years, many leaded electrical devices originally designed for through hole mounting to a substrate have been adapted or redesigned for surface mount capability.
Manufacturers and designers have been slow to develop a surface mount concept for one particular electromagnetic device called a reed switch. A reed switch uses glass enclosed, magnetically closed reeds as the contact members. The reed contacts internal to the glass extend outside opposite ends of the glass to form the leads of the device. Typically, the glass is in the shape of a cylindrical tube whereby the ends of the tube are hermetically sealed around the leads. The reeds contact each other to form a short circuit when the reed switch is placed within a sufficient magnetic field. One advantage of using a reed switch is its wireless activation by a magnetic field. Typical applications for a reed switch include proximity sensors.
The reed switch has not been readily available as a surface mountable component due to the construction of reed switch. Manufacturers of reed switches utilize the tubular nature of the glass in order to hermetically seal the glass around the cylindrical metal leads. This critical step in the reed switch manufacturing process is a result of many years of research and development. Several problems exist with trying to use a reed switch in a surface mount application. Reed switches are difficult to pick up using automatic pick and place equipment with a vision system. A vision system uses a camera to visually locate the profile of the part rather than relying on mechanical alignment. Since the reed switch body is glass, light refracting through the body makes it difficult for the vision system to define the silhouette of the part. Thus if the vision system cannot define the location of the reed switch, it cannot pick up the reed switch. Reed switches are also difficult to pick up since the cylindrical glass body is more fragile and larger than most other axial components. Another problem is that axial leads of the reed switch are not generally formed for an electrical connection to a surface mount substrate. Another problem is that the cylindrical body of the reed switch does not define an orientation for stabilizing the reed switch on the substrate. Even if the reed switch leads were formed for surface mount capability, it could roll over during handling before it gets permanently attached to the substrate. Still another problem is that the hermetic seal on the reed switch is sensitive to stresses on the lead that can break the hermetic seal. Thermal and mechanical stresses can cause the substrate to stress the leads of the reed switch.
One method which has been used to counter these problems is to mold a permanent case around the body of the reed switch. This method mounts the reed switch leads to a lead frame. The reed switch/lead frame combination is then placed in a tooling cavity in which a material is injection molded around the combination to form a pretooled shape. Although this method solves the above problems, it requires a great deal of expertise on the part of the tool makers in order that the injection flow of the material during the molding process does not damage the glass body of the reed switch. The case design and material must be carefully determined so that the glass body of the reed switch is not damaged during the thermal stresses of the operating environment of the reed switch assembly. This assembly also typically costs two to five times the solution presented by the preferred embodiment.
Others have approached the problem of surface mounting a reed switch by potting the reed switch in a plastic shell. The reed switch is placed in the plastic shell and the liquid potting material is poured into the shell. Once the potting material has cured in the shell, the reed switch leads are formed for surface mount capability. The problems with this approach include the lengthy cure time to set the potting material and the resulting high cost of the assembly.
One reed switch manufacture took the approach of designing a reed switch with a rectangular glass tube instead of a cylindrical glass tube. The leads are flat instead of round. This design is unconventional in the reed switch processing field. Although this design is effective in a surface mount application it is difficult to place using automatic pick and place with a vision system. Furthermore, from the standpoint of a buyer of surface mount reed switches, all other manufacturers have a cylindrical glass design which is difficult to use for the reasons described above.
Another method used for surface mounting a reed switch is to manually form its leads, place it on the substrate and attach it to the substrate. While this method results in a low cost surface mount reed switch, the labor and time required for this operation makes this method undesirable. This method also encourages a damaged hermetic seal in handling and the operating environment.
Therefore, there remains a need for a surface mount reed switch that can be automatically picked up and placed on a substrate, stabilized on the substrate, mounted on the substrate, buffered from the thermal and mechanical stresses on the hermetic seal and an inexpensive assembly.